The 12-notes can be divided into two groups: 7 naturals & 5 accidentals.
There are 7 natural notes in total. Natural notes are usually represented by a letter: 'A, B, C, D, E, F, G', or by their 'Solfa' name: 'Do, Re, Mi, Fa, Sol, La, Ti'.
There are 5 accidental notes in total. Each accidental have two names, sharp or flat, which is derived from its neighboring natural notes.
When an accidental note is attributed to its lower neighbor note (on its left), it will get that neighbor's name with a sharp sign '♯':
When an accidental note is attributed to its higher neighbor note (on its right), it will get that neighbor's name with a flat sign '♭':